How does a tree produce annual rings? This slide shows part of a cross section of a young conifer. The center of the tree is the pith and the outside of the tree is marked by the bark. Just
inside the bark is the vascular cambium, where cells that form rings are produced. Each year, the cambium produces phloem and xylem cells. Phloem cells are formed on the outside of the cambial layer, and transport sugars and other photosynthetic
products throughout the tree. Xylem cells are formed inside the cambial layer, and their function is to transport water from the roots up through the trunk of the tree. The cells of the phloem layers are compressed over time and become part of the
bark. The xylem cells remain rigid wood, and are the building blocks of tree rings.
There are two main types of ring producing trees, gymnosperms (non-flowering seed plants such as conifers) and angiosperms (flowering plants including trees
such as oaks, beech, and aspen). The primary cellular component of tree rings in gymnosperms is the tracheid. Tracheids are long tubular cells that make up the xylem. Tracheids formed in the beginning of the growing season are thin walled and low in
density. These cells constitute what is called the earlywood. As the end of the growing season nears, climate conditions become less conducive and growth slows. Tracheids become darker and more thick-walled, forming the latewood. Finally, when the
growth season ends, there is a marked boundary at the edge of the ring. The changes in the tracheid characteristics can be seen in the rings of the conifer shown in the slide. The earlywood portion of the ring appears lighter in color to the naked
eye, and the latewood appears dark, forming the visible light and dark bands within the annual rings.
NOAA Paleoclimatology Program
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